1. Field of the Invention
The present invention relates to a projection type image display apparatus, such as a liquid crystal projector, which modulates light from a light source by an image display element, and enlarges and projects the modulated light by a projection lens.
2. Description of the Related Art
Examples of projection type image display apparatuses having an optical system which performs polarization separation and synthesis of light from a light source are disclosed, for example, in Japanese Laid-Open Publication No. 63-39294 and Japanese Laid-Open Publication No. 8-160374.
First, a projection type image display apparatus disclosed in Japanese Laid-Open Publication No. 63-39294 is described below.
A schematic view of the structure of the apparatus is shown in FIG. 1.
A light beam emitted by a light source 12 passes through a collimating lens 13, and is separated into two orthogonal linearly polarized light beams by a polarization beam splitter 14 (henceforth referred to simply as "PBS") having a square pole shape. One of the separated polarized light beams is reflected by the PBS 14, and is incident upon a color separating prism 15. The color separating prism 15 includes a first prism 15A, a second prism 15B and a third prism 15C. On a second plane 15e of the first prism 15A, a thin dichroic interference film which reflects a blue light beam and transmits a light beam having a longer wavelength than that of the blue light beam, is vapor-deposited. There is a gap between the first prism 15A and the second prism 15B. On a plane 15f between the second prism 15B and the third prism 15C, a thin dichroic interference film which reflects a red light beam and transmits a green light beam is deposited. Therefore, when white light is incident upon an incident plane 15a, blue light included therein is reflected by the plane 15e, is totally and internally reflected by the plane 15a and heads towards an output plane 15b. Green light which passed through the plane 15f heads towards an output plane 15d. Reference numerals 16, 17 and 18 represent liquid crystal display elements for displaying a blue image, a red image and a green image respectively. Dielectric reflecting mirrors 19, 20 and 21 are provided on the back of the liquid crystal display elements 16, 17 and 18 respectively. The light beam separated by each of the prisms 15A, 15B and 15C passes through the liquid crystal display elements 16, 17 and 18 respectively, is reflected by the reflecting mirrors 19, 20 and 21 respectively, and passes through the liquid crystal display elements 16, 17 and 18 respectively again. The light beams reflected by the liquid crystal display elements 16, 17 and 18 are synthesized by the color separating prism 15, and the synthesized light beam is incident upon the PBS 14 again. The component of the incident light beam which had the polarization direction thereof modulated by the liquid crystal display elements 16, 17 and 18 in response to an image signal, passes through the PBS 14 and is projected on a screen 7 by a projection lens 6.
Next, a projection type image display apparatus disclosed in Japanese Laid-Open Publication No. 8-160374 is described below.
A schematic view of the structure of the apparatus is shown in FIG. 2.
Reference numeral 22 represents a projection light source which includes a light source and a reflecting mirror for converging the light beam emitted by the light source. The emitted light beam passes through an IR-UV cutting filter 22' which blocks unnecessary infrared radiation and ultraviolet radiation, and enters PBS 23, where the light beam is separated by a polarization separation plane 23-1 into a P polarized light beam and an S polarized light beam which are orthogonal to each other.
The S polarized light beam which was reflected by the polarization separation plane 23-1 is separated into red, green and blue light beams by dichroic mirrors 24, 25 and 26 respectively. These separated light beams are respectively incident upon a liquid crystal display panel for red 27, a liquid crystal display panel for green 28 and a liquid crystal display panel for blue 29, where the polarization direction of the light beams are rotated in response to an image signal. The resultant light beams are incident upon dichroic mirrors 30, 31 and 32 respectively, and then upon another PBS 33. The P polarized light beam passes through the PBS 33, and is enlarged and projected on a screen 7 by a projection lens 34. The P polarized light beam which passed through the PBS 23 is incident upon a liquid crystal display panel 35. The component of the incident light beam which had the polarization direction thereof modulated by the liquid crystal display panel 35, is reflected by the PBS 33, passes through the projection lens 34, and is enlarged and projected on the screen 7 as a luminance signal light beam of an image.
The projection type image display apparatuses such as the above have the following drawbacks.
As described above, these image display apparatuses use a PBS having a square pole shape as a polarization selective reflection element having a function of reflecting or transmitting light in accordance with the polarization direction thereof.
Such a PBS is made by depositing a dielectric multi-layer film on a surface of one of two glass prisms made of optical glass (e.g., BK7), and attaching the two prisms together so that the deposited surface constitutes an interface therebetween. For example, the glass prism is made by the following procedure: a large glass block is made by cooling glass which has been melted at a high temperature; a small glass block is scraped from the large glass block; and a surface of the small glass block is polished. When cooling and solidifying the glass, an annealing process is adopted in order to remove any deformations. However, it is difficult to remove the deformations completely. Furthermore, such deformations generally occur unevenly in the prism. These deformations cause birefringence in the light in the prism. Since the deformations are not uniform, the degree of birefringence and the direction of the principal axis of birefringence exhibit an uneven distribution.
In the case where a light beam is incident upon a uniaxial birefringent material, there arises a difference in the travelling speed between ordinary ray and extraordinary ray. This causes, for example, a phase contrast between the ordinary ray and the extraordinary ray in the case where a linearly polarized light beam which is diagonal to the principal axis of birefringence. Such a phase contrast adversely affects the polarization state of the incident light beam.
Problems found in the case where birefringence remains in the PBS will now be described, using the projection type image display apparatus disclosed in Japanese Laid-Open Publication No. 63-39294 as an example (see FIG. 1).
Ideally, the light beam emitted by the light source 12 is incident upon the PBS 14 and is separated by the plane of the dielectric multi-layer film into two linearly polarized light beams whose polarization directions are orthogonal to each other. Only the component of the light beam which had the polarization direction thereof rotated by the liquid crystal display panels 16, 17 and 18 in response to image information and is now orthogonal to the polarization direction of the incident light, passes through the PBS 14, and is enlarged and projected on the screen 7 by the projection lens 6.
However, in the case where birefringence remains in the prism constituting the PBS 14, while a linearly polarized light beam travels through the prism, a light beam orthogonal to the polarization direction of the incident light arises due to the birefringence. Accordingly, the component of the light beam which did not have the polarization direction thereof modulated by the liquid crystal display panels 16, 17 and 18 reaches the screen 7. In other words, light leaks even in the black display state and the contrast ratio of an image projected on the screen 7 is reduced. The above phenomenon also reduces the luminance of the image. Moreover, the unevenness of the deformations in the prism causes a noticeable problem of performance such as an occurrence of irregularity in the luminance in the image on the screen 7.
Next, problems found in the case where a PBS having a polarization separation function by means of a dielectric multi-layer film is used for a projection type image display apparatus are described with reference to FIG. 3.
The PBS including the dielectric multi-layer film has characteristics of transmitting a light beam of the polarization direction which is parallel to an incident plane (i.e., plane including the incident light and the reflected light) (i.e., P polarized light beam), and reflecting a light beam of the polarization direction which is orthogonal to the incident plane (i.e., S polarized light beam). Accordingly, different incident directions result in different polarization directions of the transmitted light beam (i.e., P polarized light beam) and the reflected light beam (i.e., S polarized light beam).
Generally, in projection type image display apparatuses, an illuminant light beam is not a perfectly collimated light beam, but a light beam having some spreading angle. When such an illuminant light beam is incident upon the PBS, the incident direction with respect to the plane of the dielectric multi-layer film varies in accordance with the spreading angle of the light beam. Accordingly, neither the light beam which passes through the PBS nor the light beam which is reflected by the PBS is completely linearly polarized to have one polarization direction. As a result, the contrast ratio is reduced in such an apparatus as disclosed in the Japanese Laid-Open Publication No. 63-39294, etc., which uses birefringence of liquid crystal molecules. Furthermore, the PBS does not allow the polarization directions of the reflected light beam and the transmitted light beam to be set at an arbitrary angle. Consequently, when the PBS is used with a display element for which the polarization direction of an incident light beam must be fixed to a predetermined direction (e.g., liquid crystal display panel), it may be necessary to change the polarization direction of the light beam from the PBS using a wave plate, and the like.
The above problems also arise in such a projection type image display apparatus as disclosed in Japanese Laid-Open Publication No. 8-160374, which uses a transmissive image display element.
Moreover, a PBS having a size of about several tens of millimeters is normally required in accordance with the size of the liquid crystal display panel. This requirement makes the production of the PBS from a glass prism expensive, which leads to an increase in the costs and the weight of the apparatus.